Generally, air filters that, use fibers as the filter medium have been used widely in conventional clean rooms. Above all else, the air filters that use glass fiber as the filter medium, such as ULPA (Ultra Low Penetration Air) filters and HEPA (High Efficiency Particle Air) filters are superior in that they can remove ultra-fine particles. For example, the ULPA filter can remove ultra-fine particles with diameters of 0.1 μm or larger.
With progressive increase in integration degree of semiconductor devices in recent years, with respect to the air contamination in clean rooms, problems not only of airborne dust but also of gaseous organic substances have come to draw attention (Fujii: “Present State of Affairs of Gaseous Contaminants and Measures for their Removal, Air Cleaning” Vol. 32, No. 3, P. 43, 1994 ).
It has been well known that the organic adsorbates on silicon wafer adsorbed from the air in the conventional clean room which contain many kinds of gaseous organic substances, would degrade the performance of semiconductor devices manufactured using the said silicon wafer. The deterioration of the semiconductor devices would be caused by a decrease in reliability of the gate oxide film (Shimazaki et al.: Proceedings of 1992 Spring Meeting of The Japan Society of Applied Physics p.686)
According to the research by the present inventors, conventional air filters generate gaseous organic substances, such as cyclosiloxanes, organic carboxylic acid esters, organic phosphoric acid esters, hydrocarbons, and phenols (Kobayashi et al.: Proceedings of the 42nd Japan Society of Applied Physics and Related Societies, No. 1, p.356, 1995). The inventors have clarified that those organic substances are generated from treatment agents used to treat fiber in the manufacture of filter medium for the air filters and the organic substances are generated also from the sealing materials for adhering the filter medium and its support frame.
The present inventors have found that a method to heat the air filter in a hot gas stream is effective to reduce the generation of organic substances from the air filter, and have filed a patent application for this method (Refer to JP-A-9-29020). The present inventors have filed a patent application for an air filter that uses a water repellent, a plasticizer, and an antioxidant that generate less organic substances as the above noted treating agent (Refer to International Publication No. WO97/04851.)
It has been well known that a a-type semiconductor can be obtained by doping of P(phosphorus) into a silicon wafer and a p-type semiconductor can be obtained by doping with B (boron) in the semiconductor device manufacturing process. If air borne phosphorus compounds or boron compounds are present in a clean room, there is a possibility that unnecessary doping occurs, so that it is required to remove those contaminants from the air of the clean room. Filter medium and sealing materials, which use fiber not containing those inorganic compounds, are also disclosed in WO97/04851.
Inorganic contaminants that give rise to problems in the manufacture of semiconductor devices include P (phosphorus) and B (boron), which have been mentioned above, and also include ammonium ion (NR4+), nitric acid ion (NO3−), sulphuric acid ion (SO42−) chlorine ion (Cl−) and so on. For example, chlorine ion or sulphuric acid ion contaminate semiconductors and reduces production yields. Ammonium ion and sulphuric ion fog up lenses that focus ultraviolet rays used as the light source in the lithographic process.
Nonetheless, in the prior art mentioned above, no study has been made of an air filter that does not generate such hazardous inorganic substances. The above-mentioned prior art has room for further improvement with respect to the reduction in organic substances that are generated in the manufacture of semiconductor devices.
The present invention has been invented with the problems of the prior art taken into consideration and has as its object to provide an air filter for trapping suspended particulate substances in the air, wherein the air filter provides better effects of suppressing the release of organic and inorganic substances that cause the trouble mentioned above than the prior-art air filter.